This invention relates to a channel-cut monochromator which can be used in a high-resolution X-ray diffractometer.
The high-resolution X-ray diffractometer has developed based on a certain technique called xe2x80x9cdouble crystal methodxe2x80x9d. With the double crystal method, a rocking curve (a graph indicating a relationship between an X-ray diffraction intensity and a diffraction angle for a certain diffraction peak) of a single crystal sample can be measured in such a manner that X-rays are diffracted by the first crystal to become a monochromatic beam and then it irradiates the sample (the second crystal). The first crystal may be usually a perfect-crystal of silicon (Si) or germanium (Ge). In the double crystal method, it is known that the angular resolution of the rocking curve becomes highest when the first crystal is the same as the sample crystal and also the diffraction plane of the first crystal is coincident with the object lattice plane of the sample crystal (i.e., the same d-value which is an interplaner spacing of lattice planes). Such an X-ray optics, in which the first crystal and the sample crystal have the same d-value, is called xe2x80x9cparallel arrangementxe2x80x9d. With this ideal arrangement, the full width at half maximum intensity (FWHM) of a measured rocking curve becomes narrowest and the shape of the rocking curve is almost coincident with the theoretically-predicted shape.
Alternatively, even when the first crystal and the sample crystal are not perfectly the same, a high resolution is obtained using the first crystal having the d-value which is nearly equal to that of the object lattice plane of the sample crystal, this optics being called xe2x80x9cquasi parallel arrangementxe2x80x9d. For example, the first crystal may be a germanium perfect-crystal with {400} reflection for obtaining the rocking curve of {400} reflection of GaAs single crystal or InP single crystal.
A large difference in d-value between the first crystal and the second crystal lowers resolution because the wavelength dispersion effect is added in a manner of convolution, so that the FWHM of a measured rocking curve becomes broader. It is desirable therefore, for measuring the highest-resolution rocking curve using the double crystal method, to select the first crystal having a d-value most closest to that of the sample crystal, the d-value of the sample crystal depending upon the kind of the crystal and the Miller indices of the object lattice plane of reflection. Accordingly, it is necessary to change the first crystal frequently in response to the kind of the sample crystal and its object Miller indices and further to conduct X-ray optical alignment whenever the first crystal is changed.
It is troublesome, however, to conduct the optical alignment again by altering the crystal arrangement based on the double crystal method and such alignment operation needs skill. Therefore, some improvements have developed for easy alignment: for example, the second crystal can be adjusted by rotation around the first crystal or the X-ray source can be adjusted by rotation around the first crystal as disclosed in, for example, Japanese patent publication No. JP 1-86100 A (1989). Such improvements, however, still need crystal exchange operation and alignment operation. Thus, such improvements still need troublesome operation and are not so efficient.
The first crystal is usually a flat crystal, but it is known that it may be a channel-cut monochromator which is manufactured by processing a groove on a monolithic single crystal block. X-rays may be diffracted plural times at the side walls of the groove to become a monochromatic parallel beam as disclosed in, for example, Japanese patent publication No. JP 9-49899 A (1997). If X-rays are diffracted even-number (e.g., two) times at the channel-cut monochromator, the output X-ray beam becomes a monochromatic parallel beam and travels in a direction parallel to the incident X-ray beam. If the output X-ray beam from the first crystal is parallel to the incident X-ray beam as mentioned above, an output X-ray beam from the first crystal after exchange of the first-crystal is to become parallel to the former output X-ray beam before the exchange. Accordingly, even when the first crystal is exchanged, it is not necessary, for alignment operation, to xe2x80x9crotatexe2x80x9d the X-ray tube or the sample unit (i.e., goniometer unit) so that the system space can be minimized, noting that a translational movement is needed for alignment operation.
The channel-cut monochromator produces an output X-ray beam which is basically identical with one from a flat crystal monochromator with single reflection. However, when X-rays are diffracted xe2x80x9cplural timesxe2x80x9d at the channel-cut monochromator, the reflection coefficient curve of the output X-ray beam has skirts with extremely reduced intensities, this being the effect of the plural times of diffraction. Also using the channel-cut monochromator, it is necessary, for obtaining the highest-resolution rocking curve, to exchange the monochromator to one having an optimum d-value in response to the kind of the sample crystal and its object Miller indices. And the exchange of the channel-cut monochromator requires in general alignment operation with a translational movement of the sample unit.
A further improvement, which requires no translational movement of the sample unit either, is to use a four-crystal monochromator as disclosed in, for example, Japanese patent publication Nos. JP 59-108945 A (1984) and JP 4-264299 A (1992). The four-crystal monochromator is composed of two channel-cut monochromators arranged to be mirror-symmetrical. The four-crystal monochromator produces a highly-monochromatic and highly-parallel output X-ray beam which is also on the extension line of the incident X-ray beam. Using the X-ray beam produced by the four-crystal monochromator, a high-resolution rocking curve of a sample is always obtained without depending upon the kind of the sample crystal and its object Miller indices. Using the four-crystal monochromator however, the output X-ray beam inadvantageously has a very low intensity which would be about one hundredth of that produced by the double crystal method. Thus the use of the four-crystal monochromator has some problems: (1) it requires a high-power X-ray source which is expensive; and (2) it takes a long time to measure the rocking curve for accumulating the intensity. Therefore, the four-crystal monochromator would be limited to have only such reflecting surfaces that its Miller indices can produce a high-intensity X-ray beam.
Next, the state of art in X-ray analysis using a high-resolution X-ray diffractometer is described below. As a thin film technique spreads, object samples of the high-resolution X-ray diffractometer spread from the conventional bulk crystals toward film crystals on substrates. The crystal state of the film is in variety and classified to (1) a perfect epitaxial layer (pseudomorphic layer), (2) an epitaxial layer in which dislocations occur in a boundary between a substrate crystal and the epitaxial layer for strain relaxation, (3) an epitaxial layer having an orientation distribution (mosaicity), (4) a polycrystalline thin film having strong preferred orientation, (5) a polycrystalline thin film having no preferred orientation and (6) an amorphous thin film. Under the circumstances, the high-resolution X-ray diffractometer has been expected to have various functions so as to measure not only the rocking curves mentioned above but also reflection coefficient (near the total reflection region with glancing incident angles) and polycrystalline film X-ray diffraction.
Therefore, various incident optical systems have to be prepared to regulate, according to the sample state, the parallelism and the wavelength range of X-rays which are incident on a sample. Such an incident optical system may be a module-type incident optical unit which can be exchanged for another or an incident optical system which can be switched to another state without removing a crystal as disclosed in, for example, Japanese patent publication No. JP 9-49811 A (1997). However, the exchange of the module-type incident optical unit is expensive because various optical units must be prepared and exchanged and the fine tuning after the exchange would be troublesome.
The switchover of the incident optical system disclosed in Japanese patent publication No. JP 9-49811 A (1997) can select one of the following four incident optical systems.
(1) Taking out the direct beam. That is, an X-ray beam is not reflected by the crystal and passes through as it is. This incident optical system is usable mainly for polycrystalline thin film diffraction.
(2) A channel-cut monochromator optical system. This system has a channel-cut crystal using Ge {220} reflection by which X-rays are reflected two times, so that CuKxcex12 is removed and CuKxcex11 only is taken out. This incident optical system is usable mainly for reflection coefficient measurement.
(3) A high-intensity mode of four-crystal monochromator. This incident optical system has a combination of two channel-cut monochromators using Ge {220} reflection and usable for rocking curve measurement of epitaxial layers for example.
(4) A high-resolution mode of four-crystal monochromator. This incident optical system has a combination of two channel-cut monochromators using Ge {440} reflection. Since this system has very high resolution, it is usable for rocking curve measurement of perfect epitaxial layers for example.
The switchover of the four incident optical systems is carried out by CPU control in a manner that some adjusting members are adjusted to the predetermined value, this switchover operation being easy. However, if a new incident optical system other than the four systems would be desired, an optional crystal is required. The optional crystal can be installed as explained below. First, a mechanical clamp is loosed to remove a crystal along with its holder block from the incident optical system. Then, a holder block having the optional crystal is mechanically clamped, noting that the position and the angle of the newly clamped crystal are not always accurate because they depend upon the clamping force and so on. Accordingly, it is necessary to re-adjust the optical system or at least to set at the former adjusted value and carry out fine tuning thereafter, this operation being troublesome.
The optional crystal may be one of the following monochromators:
(1) An asymmetrical-reflection channel-cut monochromator which condenses the beam width. This monochromator is used for increasing the X-ray beam intensity. When the asymmetrical reflection with which the output X-ray beam width becomes narrower than the incident X-ray beam width is used, an output X-ray beam intensity per unit width is increased but the angular resolution is lowered as compared with the symmetrical reflection. This monochromator is used for measurement of X-ray reflection coefficient of a thin film. The X-ray reflection coefficient method can evaluate a thickness and a density of a thin film and a density of a surface or a boundary. When the method is applied to a very thin film to make analysis with high accuracy, it is necessary to measure intensity variation of X-rays reflected by a thin film in an angular range from a grazing incident angle to about ten degrees with a dynamic range of eight figures or more. This requires a narrow-width, high-intensity monochromatic X-ray beam which is obtained using the asymmetrical-reflection channel-cut monochromator condensing the beam width.
(2) An asymmetrical-reflection channel-cut monochromator which expands the beam width. This monochromator uses such an asymmetrical reflection that the output X-ray beam width becomes broader than the incident X-ray beam width in contrast to the above-mentioned beam-width condensation. An output X-ray beam intensity per unit width is decreased but the angular resolution is advanced. This monochromator is used for X-ray topography because an image area covered by one measurement is large.
(3) A channel-cut monochromator with quasi parallel arrangement. That is, the d-value of the channel-cut crystal is almost identical with that of a sample crystal. Using this monochromator, a high-intensity, high-resolution rocking curve is obtained. Since there is a tendency that the thickness of an epitaxial layer becomes thinner and thinner, an intensity of diffracted X-rays from the epitaxial layer would be not enough in some cases, this monochromator being usable for such cases. Furthermore, this monochromator is also usable for the following cases for which the four-crystal monochromator is not usable because of its weak intensity of the output X-ray beam. (a) When a sample crystal is curved, an X-ray irradiation width on the sample should be narrowed to avoid influence of the curve. In this case, the X-ray intensity is low. (b) When a small region of a selectively-deposited layer is analyzed, it is required to narrow an X-ray beam in width and height and to aim the X-ray beam at the target region. In this case, the X-ray beam is narrowed to have a cross-section of about 20 micrometers wide and 50 micrometers high, the X-ray intensity being small. (c) When an epitaxial thin film crystal is analyzed, a certain evaluation method has been established such that a sample is measured using two or more diffraction vectors. For example, for investigating whether or not strain relaxation occurs in the boundary, there is observed usually not only {400} symmetrical reflection but also {511} and {422} asymmetrical reflection. The all kinds of reflection can be measured satisfactorily with a high-resolution using the four-crystal monochromator. However, in case that the weak intensity becomes an issue as described in the above terms (a) and (b), the four-crystal monochromator would be insufficient and provides no efficient measurement. After all, in case that the weak intensity becomes an issue, the xe2x80x9cdouble crystal methodxe2x80x9d is effective in which both a high-resolution and a high-intensity are obtained with the use of a channel-cut monochromator which is in a quasi parallel arrangement for the object lattice plane of a sample crystal, noting that it requires exchange of the monochromator crystal and an alignment operation thereafter.
As has been described in detail, in the field of the high-resolution X-ray diffractometer, the double crystal method is effective for obtaining both a high resolution and a high intensity. Using the double crystal method, however, for measurement under various conditions, it is necessary to exchange the incident optical unit or switch the incident optical system, it being troublesome.
It is an object of the invention to provide a channel-cut monochromator which is manufactured by processing a plurality of grooves on a common crystal block and can be rotated to switch the reflection Miller indices or select symmetrical or asymmetrical reflection.
A channel-cut monochromator according to the invention comprises at least two, preferably three or more, kinds of reflecting surface pairs processed on a common single crystal block. Each reflecting surface pair has a first and a second reflecting surfaces between which an X-ray beam is reflected even-number times (typically two times, or four or six times being usable). This channel-cut monochromator can be rotated around an axis of rotation perpendicular to a reference plane so as to switch the reflecting surface pair which reflects X-rays. Any one of the reflecting surface pairs is composed of two reflecting surfaces perpendicular to the reference plane, and an X-ray beam incident on any reflecting surface pair or its extension line is tangent to a common imaginary circle whose center coincides with the axis of rotation. With this structure, the switchover of the reflecting surface pair is accomplished by only rotation of the channel-cut monochromator around its axis of rotation, so that X-ray beams reflected by various Miller indices can be taken out selectively.
The channel-cut monochromator may have a direct path through which an X-ray beam passes in no contact with any reflecting surface. The X-ray beam passing through the direct path passes through the channel-cut monochromator so as to be tangent to the imaginary circle. With the direct path, an X-ray beam can pass through as it is without escaping the channel-cut monochromator from the optical axis.
The channel-cut monochromator may have more preferably five or more reflecting surface pairs. For example, the channel-cut monochromator may be made of silicon or germanium single crystal and may have at least five kinds of reflecting surface pairs for {220}, {400}, {422}, {511} and {111} reflection. The reflecting surface pair for {220} reflection is also usable for taking out an X-ray beam of {440} reflection. The {220} and {440} reflection is usable for the four-crystal monochromator. An X-ray beam from {111} and {220} reflection has comparatively a high intensity and is usable for measurement of reflection coefficient. On the other hand, an X-ray beam from {400}, {422} and {511} reflection is usable for a parallel arrangement or quasi parallel arrangement in the double crystal method.
The channel-cut monochromator may have at least one reflecting surface pair having one or two asymmetrical reflecting surfaces. The asymmetrical reflecting surface may be a type of condensing an X-ray beam width or a type of expanding an X-ray beam width. Only one of the two reflecting surfaces composing a reflecting surface pair may be asymmetrical or both of them may be asymmetrical.
Since the channel-cut monochromator of the invention is made of a common single crystal block having at least two kinds of reflecting surface pairs and is rotated to switch the reflecting surface pair, various X-ray beams reflected by various Miller indices can be taken out selectively. Specifically, some kinds of reflecting surface pairs having Miller indices suitable for measurement of reflection coefficient or measurement using the four-crystal monochromator and other kinds of reflecting surface pairs having Miller indices which require the double crystal method both can be formed on the common single crystal block, so that these Miller indices can be switched easily. The switchover of the Miller indices can be carried out by only rotation of the channel-cut monochromator and, if necessary, the translational movement of the sample unit, so that the switchover operation is easy.